Electrochemical Synthesis of Elongated Noble Metal Nanoparticles, such as Nanowires and Nanorods, on High-Surface Area Carbon Supports

ABSTRACT

Elongated noble-metal nanoparticles and methods for their manufacture are disclosed. The method involves the formation of a plurality of elongated noble-metal nanoparticles by electrochemical deposition of the noble metal on a high surface area carbon support, such as carbon nanoparticles. Prior to electrochemical deposition, the carbon support may be functionalized by oxidation, thus making the manufacturing process simple and cost-effective. The generated elongated nanoparticles are covalently bound to the carbon support and can be used directly in electrocatalysis. The process provides elongated noble-metal nanoparticles with high catalytic activities and improved durability in combination with high catalyst utilization since the nanoparticles are deposited and covalently bound to the carbon support in their final position and will not change in forming an electrode assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of Copending U.S. patent applicationSer. No. 13/624,149, filed Sep. 21, 2012, which is aContinuation-In-Part (CIP) of U.S. patent application Ser. No.12/603,216, filed Oct. 21, 2009, U.S. Pat. No. 8,699,207, which claimsthe benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No.61/107,048, filed Oct. 21, 2008, U.S. patent application Ser. No.13/624,149 is also a Continuation-In-Part (CIP) of PCT PatentApplication No. PCT/US2011/43901, filed Jul. 13, 2011, which claims thebenefit under 35U.S.C. 119(e) of U.S. Provisional Application No.61/364, 040, filed Jul. 14, 2010, U.S. patent application Ser. No.13/624,149 also claims the benefit, under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/537,814, filed Sep. 22, 2011. The contentof all above-referenced applications is incorporated herein in itsentirety.

STATEMENT OF GOVERNMENT RIGHTS

The present invention was made with government support under contractnumber DE-AC02-98CH10886 awarded by the U.S. Department of Energy. TheUnited States government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to nanoparticles and methods for theirmanufacture, in particular, the present invention relates tonanometer-scale particles produced by electrochemical deposition onhigh-surface carbon nanoparticles. The invention also relates tocontinuous and nonporous core-shell electrocatalysts for the O₂reduction reaction. The invention also relates to the incorporation ofsuch nanoparticles in energy conversion devices.

BACKGROUND

Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), andrelated alloys are known to be excellent catalysts. When incorporated inelectrodes of an electrochemical device such as a fuel cell, thesematerials function as electrocatalysts since they accelerateelectrochemical reactions at electrode surfaces yet are not themselvesconsumed by the overall reaction. Although noble metals have been shownto be some of the best electrocatalysts, their successful implementationin commercially available energy conversion devices is hindered by theirhigh cost and scarcity in combination with other factors such as asusceptibility to carbon monoxide (CO) poisoning, poor stability undercyclic loading, and the relatively slow kinetics of the oxygen reductionreaction (ORR).

A variety of approaches has been employed in attempting to address theseissues. One well-known approach involves increasing the overall surfacearea available for reaction by forming metal particles withnanometer-scale dimensions. Loading of more expensive noble metals suchas Pt has been further reduced by forming nanoparticles from alloyscomprised of Pt and a low-cost component. Still further improvementshave been attained by forming core-shell nanoparticles in which a coreparticle is coated with a shell of a different material which functionsas the electrocatalyst. The core is usually a low-cost material which iseasily fabricated whereas the shell comprises a more catalyticallyactive noble metal. An example is provided by U.S. Pat. No. 6,670,301 toAdzic, et al. which discloses a process for depositing a thin film of Pton dispersed Ru nanoparticles supported by carbon (C) substrates.Another example is U.S. Pat. No. 7,691,780 to Adzic, et al. whichdiscloses platinum- and platinum alloy-coated palladium and palladiumalloy nanoparticles. Each of the aforementioned U.S. patents isincorporated by reference in its entirety as if fully set forth in thisspecification.

One approach for synthesizing core-shell particles with reduced noblemetal loading and enhanced activity levels involves the use ofelectrochemical routes which provide atomic-level control over theformation of uniform and conformal ultrathin coatings of the desiredmaterial on a large number of three-dimensional nanoparticles. One suchmethod involves the initial deposition of an atomic monolayer of a metalsuch as copper (Cu) onto a plurality of nanoparticles by underpotentialdeposition (UPD). This is followed by galvanic displacement of theunderlying Cu atoms by a more noble metal such as Pt as disclosed, forexample, in U.S. Pat. No. 7,704,918 to Adzic, et al. Another methodinvolves hydrogen adsorption-induced deposition of a monolayer of metalatoms an noble metal particles as described, for example, by U.S. Pat.No. 7,507,495 to Wang, et al. Each of the aforementioned U.S. patents isincorporated by reference in its entirety as if fully set forth in thisspecification.

Although each of these approaches has been successful in providingcatalysts with a higher catalytic activity and reduced noble metalloading, still further improvements in both the durability andmass-specific catalytic activity are needed for electrochemical energyconversion devices to become reliable and cost-effective alternatives toconventional fossil fuel-based devices.

One issue relating to the manufacture of conventional singlenanoparticles or core-shell nanoparticles is the formation of MOHspecies on the surface of these nanoparticles that inhibit oxygenreduction reaction (ORR). A large number of low-coordination atoms onthe surface of the nanoparticles is particularly prone to such oxidationand to gradual dissolution of the electrocatalyst over time. Withcontinued operation, this tends to reduce the catalytic activity of theelectrocatalyst and cause damage to the electrolyte membranes containedwithin a typical energy conversion device, thereby reducing its chargestorage and energy conversion capabilities. One approach forsynthesizing nanoparticles with a smaller number of low coordinationsites and higher specific activity for the O₂ reduction reaction is tosynthesize nanowires and nanorods, which tend to have smoother surfaces.However, the conventional synthesis methods tend to use capping agentsand surfactants to prevent agglomeration and facilitate growth of thedesired shape and take hours or days to complete. An exemplary processis provided in Song et al. (Nano Letters, 2007, 7(12), 3650), which isincorporated herein by reference in its entirety. Unfortunately,removing these surfactants is quite difficult and inevitably causesincrease of the particles' thickness, breaking of the nanowires, and anincrease of the number of low coordination atoms.

There is therefore a continuing need to develop manufacturing methodsthat would avoid the use of capping agents and surfactants and can becompleted within minutes, necessary for large-scale and cost-effectiveprocesses suitable for commercial production and incorporation inconventional energy production devices.

SUMMARY

In view of the above-described problems, needs, and goals, the inventorshave devised embodiments of the present invention in which methods foranisotropic growth of elongated noble-metal nanoparticles, preferablypalladium (Pd) nanowires and nanorods, are provided. The disclosedmethod is directed to the formation of the elongated nanoparticles byelectrochemical deposition of metal on functionalized high surface areacarbon supports. Preferably, the high surface area carbon supportcomprises carbon nanoparticles that can be functionalized by oxidation.The nanoparticles produced by this method can be directly used inelectrocatalysis or can be covered by a thin shell layer over the outersurface of the nanoparticle cores. The manufacturing process is simpleand cost-effective taking minutes in comparison with hours or days inchemical routes without the use of any capping agents or surfactants,which would require additional steps to remove. The process alsoprovides nanoparticles with high catalytic activities and improveddurability in combination with high catalyst utilization since thenanoparticles are deposited in their final position and will not changein forming a membrane electrode assembly.

The nanoparticles may be comprised of a single noble metal, but maycomprise a plurality of metal elements or components selected from noblemetals. When more than one noble metal is used, the nanoparticle alloyis preferably a homogeneous solid solution, but it may also havecompositional nonuniformities. The noble metal preferably includes atleast one of palladium (Pd), iridium (Ir), rhenium (Re), ruthenium (Ru),rhodium (Rh), osmium (Os), gold (Au), and platinum (Pt), either alone oras an alloy. The preferred metal used to generate nanowires or nanorodsin the disclosed method is palladium (Pd). However, if the disclosedmethod is used to prepare core-shell nanoparticles, the preferred metalused for the core is palladium (Pd) and the preferred metal used for theshell is platinum (Pt). The nanoparticles that form a core and a morenoble metal shell are described in, for example, U.S. Pat. No. 7,704,918to Adzic, et al. and U.S. Pat. No. 7,507,495 to Wang, et al., which areincorporated herein by reference each in its entirety as if fully setforth in this specification.

In a preferred embodiment, the nanoparticles are formed on a carbonsupport, preferably high surface area carbon such as carbonnanoparticles, by a process which involves (i) functionalizing thecarbon support to generate functional groups on the surface of thecarbon support, (ii) forming a thin film of the functionalized carbonsupport on an electrode, (ii) preparing a pH-buffered solutioncontaining a salt of a metal, (iv) immersing the electrode in thepH-buffered solution, (v) applying a first potential pulse to reduce themetal and nucleate metal nanoparticles on surfaces of the functionalizedcarbon support, and (vi) applying a second potential pulse to increasethe size of the nucleated metal nanoparticles.

The density of nanoparticles is largely determined by the initialnucleation rate that increases with making the potential more negative.Hence, the first potential is typically used to control the density ofnanoparticles and is often much lower than an equilibrium potential ofthe metal or the onset deposition potential for the metal ions in thesolution. However, it was discovered that the nucleation density canalso be increased by functionalizing the carbon support, for instance byoxidation. The process generates carboxylic, carbonyl, phenol, orlactone functional groups that interact with metal salt molecules duringnucleation. Without being bound by theory, it is believed that thisinteraction allows for the metal salt molecules to be adsorbed morestrongly and in greater amounts than on the non-functionalized carbonsupport.

In a particularly preferred embodiment, the functionalized high surfacearea support comprises carbon black nanoparticles, e.g., Vulcan® XC-72R(Cabot Corp., Boston, Mass). The carbon black is preferablyfunctionalized by treating carbon black with an oxidizing agent such asconcentrated nitric acid under ambient conditions for 1 to 20 hours. Thefunctionalized carbon black is subsequently purified and deposited in athin layer or film on the surface of an electrode. The electrode ispreferably a rotating disc electrode (RDE) that is rotated at constantrotational rate of 1000 to 2000 rpm, with about 1600 rpm beingpreferred. The solution for electrodeposition may comprise 0.05 mM to 50mM PdCl₂ or Pd(NH₃)₄Cl₂ and 0.1 M NaCl while the first potential isbetween −0.4 V and −0.3 V and the second potential is between −0.1 V and−0.25 V versus a Ag/AgCl (3 M CaCl) reference electrode. However, it isalso envisioned that the first potential may be the same as the secondpotential although both potentials are lower than the equilibriumpotential of the metal.

The elongated nanoparticles produced following the disclosed methodeither by themselves or as part of core-shell nanoparticles areparticularly advantageous when incorporated into one or more electrodesof an energy conversion device. The structure of such a device comprisesat least a first electrode, a conducting electrolyte, and a secondelectrode. At least one of the first or second electrodes comprisesmetal nanoparticles preferably made from Pd nanorods or nanowires, orcore-shell nanostructures preferably made from a Pd core having ananorod or nanowire morphology and a continuous and nonporous shell madefrom Pt. In a preferred embodiment, the nanoparticles incorporated intoan energy conversion device have an external diameter of 3 nm to 9 nm.

The production of nanoparticles directly on the functionalized highsurface area support therefore permits deposition of nanoparticles attheir final position on the electrode surface that will not change informing the membrane electrode assembly, thereby maximizing theavailable catalytically active surface area and improving stability. Theuse of the disclosed method facilitates a more efficient expedient, andcost-effective process for producing catalytic electrodes forelectrochemical energy conversion in devices such as fuel cells andmetal-air batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing the sequence of steps followed in ageneralized method of forming electrocatalytic nanoparticles on thehigh-surface area support with exposed functionalized groups accordingto the present invention.

FIG. 2 is a flowchart showing the sequence of steps followed in anexemplary method of forming electrocatalytic nanoparticles on the carbonnanoparticles functionalized by oxidation according to the presentinvention.

FIG. 3A is a plot showing current transients in response to an appliedpotential for Pd deposition on oxidized carbon nanoparticles as afunction of time.

FIG. 3B is a plot showing the integrated charge associated withdeposited Pd as a function of time.

FIG. 4A is a TEM image of the nanowires prepared on carbonnanoparticles.

FIG. 4B is a TEM image of the nanowires prepared on carbonnanoparticles.

FIG. 5 is a plot showing polarization curves for the oxygen reductionreaction (ORR) on Pt monolayer-coated Pd nanowires.

DETAILED DESCRIPTION

The generalized process for forming elongated nanoparticles according tothe disclosed method is described by the flowchart shown in FIG. 1. Themethod not only provides a deposition process that takes minutes incomparison with hours or days in comparable chemical routes, but alsodoes not require the use of capping agents, surfactants, or otherorganic compounds to facilitate a heightened catalytic activity.Surfactants nave generally been used to control the particle size and toattain a higher particle yield. However, the inclusion of an organicmaterial during particle synthesis significantly lowers the catalyticactivity of the particles. Removal of the organic material using PriorArt techniques requires the use of additional washing and/or heatingprocesses which increase both the number of processing steps and theoverall cost. Furthermore, even with the appropriate cleaning steps, aresidual organic layer typically remains on the surfaces of thenanoparticles. Finally, the nanoparticles can be deposited at thedesired position on the electrode that will not change in forming themembrane electrode assembly, thus allowing the most efficientutilization of the catalyst.

As illustrated in FIG. 1, the process involves the initial production offunctionalized support material in step S1, preferably by oxidizingcarbon nanoparticles. This is followed by formation of elongatednanoparticles of a first material M1 by electrodeposition in step S2.Preferably M1 is Pd, although other noble metals can be successfullyused, such as iridium (Ir), rhenium (Re), ruthenium (Ru), rhodium (Rh),osmium (Os), gold (Au), and platinum (Pt), either alone or as an alloy.The morphology of the generated structure is preferably a nanowire or ananorod; although it could also be a nanosheet, a nanotube, or ananocone. At this stage of the process, the generated nanostructures canbe used for electrocatalysis or as a nanoparticle core in generation ofa core-shell nanostructure. For core-shell nanostructures, step S2 isfollowed by the formation of an ultrathin film of a second material M2onto the surfaces of the nanoparticle cores in step S3. Thus, theprocess can generate two distinct types of nanoparticles: (1) elongatednanoparticles made from M1 and (2) elongated core-shell nanoparticlesmade from M1/M2.

The particular methods used to form the functionalized support materialhaving a high surface area in step S1, the metal nanoparticles on thesurface of the functionalized support by electrodeposition in step S2,and the core-shell nanoparticles by displacement in step S3 are notintended to be limited to any particular process without deviating fromthe spirit and scope of the present invention. Rather each of theaforementioned steps may be accomplished using any of several processeswhich are well-known in the art. For example, steps S2 and S3 have beendisclosed in PCT Pat. App. No. PCT/US2011/43901, which is incorporatedherein by reference in its entirety. Throughout this specification, thenanoparticles and processes for their manufacture will be describedusing one or more noble metals due to the advantages provided by theiruse as electrocatalysts and/or catalysts in general.

I. Nanoparticle Synthesis on Functionalized Support

Initially a support having suitable surface area, preferably made fromcarbon, is treated by any technique which is well-known in the art togenerate a plurality of surface-bound functional groups. The support maycomprise, but is not limited to, solid carbon nanoparticles, porousmacro-scale carbon powder, porous carbon nanoparticles, carbonnanotubes, carbon paper, carbon cloth, reticulated glassy carbon, andgraphene. In a preferred embodiment as illustrated in FIG. 2, if thesupport (7) comprises carbon-based nanoparticles, it may befunctionalized by oxidation in a strung acid for 1-20 hours underambient temperature and pressure to generate surface exposed functionalgroups (8). The functional groups may include carboxylic (R—COOH),carbonyl (RC═OR), phenol (Ph—OH), lactone (ROC═OR), and a combination oftwo or more these functional groups. In preferred embodiment, at least30% of the support material (7) surface is functionalized. In the morepreferred embodiment, at least 50% of the support material (7) surfaceis functionalized. in the most preferred embodiment, at least 70% of thesupport material (7) surface is functionalized. The duration ofoxidation by acid treatment may be adjusted to attain the desiredfunctionalization coverage and distribution, e.g., 1 hour, 2 hours, . .. 19 hours, 20 hours, and anything therebetween. The acid used forcarbon material oxidation may include, but is not limited to, about5-70% nitric acid, 5-95% sulfuric acid, as well as perchloric andphosphoric acids.

It is to be understood, however, that the invention is not limited tothe generation of functional groups on the surface of the supportmaterial by acid-dependent oxidization and may include other approacheswell-known in the art as long as these approaches produce a largecoverage of functional groups on the surface of the support material. Itis also to be understood that the invention is not limited to carbon andmay include other support materials or combinations of materials whichare well-known in the an as long as these materials can producefunctional groups on their surface. The support may be comprised of asingle element or material or, in an alternate embodiment, it may be acomposition of materials. The support preferably has a high surface areaper unit mass, which may be attributed to its size.

Preferably, the support is a powder of carbon nanoparticles, althoughthe support may also include carbon nanotubes, carbon paper, porousmacro-scale carbon powder, porous carbon nanoparticles, carbon cloth,reticulated glassy carbon, and graphene. Carbon black sold under thetrade name Vulcan® and available from Cabot Corp. (Boston, Mass.), andin particular Vulcan® XC-72R, is one example of a suitable powder ofcarbon nanoparticles. However, such high surface area, support is notlimited to homogeneous solid carbon, but may also have added surfaceporosity, e.g., Vulcan® P. Suitable carbon powder supports arepreferably spherical or spheroidal with a size ranging from 2 nm to 100nm along at least one of three orthogonal dimensions, and are thusnanometer-scale particles. It is to be understood, however, that theparticles may take on any shape, size, or structures which include, butare not limited to, branching, conical, pyramidal, cubical, cylindrical,mesh, fiber, cuboctahedral, icosahedral, and tubular nanometer-scaleparticles. The carbon support particles may be agglomerated ordispersed, formed into ordered arrays, fabricated into an interconnectedmesh structure, either formed on a supporting medium or suspended in asolution, and may have even or uneven size distributions. The particleshape and size is preferably configured to maximize surfacefunctionalization activity, for example, by oxidation. In a preferredembodiment the nanoparticle cores have external dimensions of less than100 nm along at least one of three orthogonal directions. Throughoutthis specification, the carbon support particles will be primarilydisclosed and described as particles which are substantially sphericalin shape.

Once the functional bred high surface area support material is prepared,the elongated solid nanoparticles or nanoparticle cores are formed,preferably via a covalent bond, on their surface by pulseelectrodeposition. This method involves initially preparing a thin filmof already functionalized carbon powder (7), preferably nanometer-scale,on a glassy carbon electrode (3) as shown in FIG. 2. Prior approacheshave typically used a thin layer of Nafion, a polymer membrane, to affixthe carbon powder onto the surface of glassy carbon electrodes, carbonpaper, and carbon cloth. However, in the disclosed method Nafion is notneeded since a thin film of carbon powder is formed directly onto theglassy carbon electrode. A pH-buffered solution (5) containing a salt ofthe metal to be reduced is then produced. The carbon-coated electrode(3) is then immersed in the solution (5) containing a salt of the metalto be reduced. A three-electrode electrochemical cell (1) may be used toreduce the metal on the surface of the functionalized support byapplying a first potential pulse to reduce the metal ions from solutionand nucleate metal nanoparticles on the surfaces of the carbon support(7). The first potential pulse may be followed by a second potentialpulse whose duration is used to control the final size of the nucleatingnanoparticles. The electrochemical cell (1) used for nucleation andgrowth of nanoparticles typically has a counter electrode (2), areference electrode (4), and an external power supply (6).

The first potential pulse is used to control the nucleation rate whereasthe second potential pulse is used to drive subsequent growth of thenucleated nanoparticles. By using two separate potential pulses, boththe nucleation density and the size of nanoparticles produced can beindependently controlled by the duration of the pulses at the twopotentials. Without being bound by any particular theory, it is believedthat the nucleation density can also be controlled by the presence orabsence of functional groups (8) on the surface of the support particles(7). Thus, a greater number of functional groups (8) on the surface ofthe support (7) will increase the nucleation density of nanoparticlesduring the first potential pulse, thereby impacting the growth mechanismof nanoparticles, e.g., anisotropic growth of nanowires and nanorods.

In one embodiment, the first potential may range from −0.5 V to −0.3 Vwhile the second potential may range from −0.25 V to −0.1 V. In apreferred embodiment, in order to facilitate anisotropic growth ofnanowires and nanorods on the surface of the functionalized supportmaterial, the first potential may range from −0.32 V to −0.3 V while thesecond potential may range from −0.22 V to −0.15 V; it is even morepreferable to set the first potential to about −0.21 V while setting thesecond potential to about −0.2 V. All potential pulses are typicallymeasured versus a Ag/AgCl (3 M NaCl) reference electrode, but may bedetermined using other reference electrodes provided in methods known inthe Art.

When forming elongated nanoparticles on the functionalized carbonsupport from a solution containing noble metal ions, the pH of thesolution is preferably less than 2. A suitable noble metal solution forproducing Pd nanoparticles, nanowires, or nanorods may comprise, forexample, 5 mM PdCl₂ and 0.1 M NaCl in water. However, the nanoparticlesprepared by the disclosed method may also include nanoparticles madefrom such noble metals as Pt, Ir, Ru, Os, Au, or Re by immersion in asolution comprising one or more of K₂PtCl₄, PdCl₂, IrCl₃, RuCl₃, OsCl₃,OsCl₃, HAuCl₃, or ReCl₃, respectively. Pulse potential deposition of Pdnanoparticles may then proceed by applying a first potential pulse inthe range of −0.5 V to −0.3 V followed by a second potential pulse inthe range of −0.25 V to −0.20 V. The pulse durations may be adjusted toattain the desired nucleation density and size distribution.

It is to be understood that the methods of forming the nanoparticlesdescribed above are merely exemplary. Any of a plurality of alternativeelectrodeposition methods which are well-known in the art and which arecapable of forming nanoparticles with the desired shape, size, andcomposition may be employed. The key aspect is that the surface of thesupport material used for electrodeposition is functionalized toincrease the nucleation density of nanoparticle growth. It isparticularly preferred that the surface functionalized of thecarbon-based support material he adjusted to maximize the nucleation ofthe growing nanoparticles.

Once nanoparticles having the desired shape, composition, and sizedistribution have been fabricated on the surface of the functionalizedsupport material an ultra thin shell layer may then be formed togenerate a core/shell nanostructure by galvanic displacement. Theparticular process used to form the shell layer is not intended to belimited to any particular process, but is generally intended to be suchthat it permits formation of ultra thin films having thicknesses in thesubmonolayer-to-multilayer thickness range that can serve as core-shellnanoparticles tor electrocatalytic applications.

In the exemplary embodiment, the process tor forming a shell layeroccurs by galvanic displacement when the nanoparticle cores are immersedinto a solution comprising a salt of a more noble metal. Since the saltis more noble than the core material, an irreversible and spontaneousredox reaction in which core surface atoms are oxidized and replaced bythe more noble ions contained in solution occurs. The final thicknessand surface coverage of the resulting noble metal shell layer can becontrolled by varying process parameters such as the concentration ofthe noble metal salt and the duration of the immersion in solution, forpurposes of this specification, a process to generate core/shellnanoparticles is described in detail in PCT App. No. PCT/US2011/43901filed on Jul. 11, 2011, which is incorporated herein by reference in itsentirety.

In one exemplary embodiment, galvanic displacement is performed byimmersing the nanoparticle cores in a solution comprising 0.05 mM to 5mM K₂PtCl₄ to product a Pt shell layer. In another exemplary embodimenta PdAu shell layer may be formed by immersing the particles cores in asolution comprising 0.5 mM Pd(NH₃)₄Cl₂ and 0.025 mM HAuCl₃. In two otherexemplary embodiments Ru and Ir shell layers may be formed by immersingthe nanoparticle cores in a solution comprising 1 mM RuCl₃ and IrCl₃,respectively. In the preferred embodiment, the nanoparticles cores aremade from Pd and the nanoparticle shells are made from Pt. The durationof exposure in each of the above exemplary metal salts is set to obtainthe desired thickness of the shell layer.

II. Energy Conversion Devices

In a preferred application, the nanoparticles as described above may beused directly or in a core/shell system as an electrode in an energyconversion device such as a fuel cell. The use of these nanoparticlesadvantageously provides a smooth surface that gives rise to anenhancement of the ORR activity and serves as an excellent core formonolayer Pt shell deposition. Use of these nanoparticles in a fuel cellis, however, merely exemplary and is being used to describe a possibleimplementation of the nanoparticles. Implementation as a fuel cellelectrode is described, for example, in U.S. Pat. No. 7,691,780 toAdzic. It is to be understood that there are many possible applicationsfor the nanoparticles which may include, but are not limited to, chargestorage devices, applications which involve corrosive processes, as wellas various other types of electrochemical or catalytic devices.

III. Example

An exemplary embodiment of the nanoparticles will be described in detailwith reference to FIGS. 3-5. In this embodiment, carbon powder (˜60μg/cm² Vulcan® XC-72R) was initially dispersed in 15 ml H₂O bysonication in an ice-mixed ultrasonic bath and oxidized by stirring in aconcentrated nitric acid (HNO₃) for 1-20 hours at room temperature andatmospheric pressure, with about 3 hours being preferable. The producedfunctionalized carbon powder (which also may be referred to as anoxidized carbon powder) was washed with deionized water in order toremove the excess oxidizing agent, filtered using a Millipore® membranefilter, and dried at 60° C. overnight in a Fisher Laboratory dryer. Thefunctionalized carbon powder was suspended in water to form a uniformslurry (˜1 mg in 1 ml H₂O). An amount equal to 15 μl of this uniformslurry was transferred to a glassy carbon rotating disk electrode havinga diameter of 0.5 cm.

After drying in air, the carbon thin-film electrode was brought into anArgon (Ar)-saturated 5 mM PdCl₂ and 0.1 M NaCl solution. The Pdnanoparticles were generated by applying a single potential pulse at−0.5 V (vs. Ag/AgCl, 3 M NaCl) for 5 ms followed by a pulse at −0.21 Vfor 60 s. The Pd nanoparticles were produced up to 10 mC integratedcharge that corresponds to about 28 μg/cm² of Pd. Within 5 minutes (˜300sec), the open-circuit potential rose to a stable value as shown in FIG.3A. The integrated charge over the deposition time associated withdeposited Pd was 10 mC.

Formation of a Pt shell layer was accomplished by transferring therotating disk electrode into a deaerated K₂PtCl₄ solution in the sameAr-filled compartment. Pt ions in solution were reduced by metallic Cuvia the reaction Cu+Pt²⁺→Cu²⁺→Pt with the amount controlled by theconcentration of K₂PtCl₄ (0.1 mM to 1 mM) and the duration of galvanicreplacement (3 to 30 minutes). After the electrode was immersed for apredetermined period of time, it was removed from solution and rotatedin pure water to remove residual metal ions. Sample high resolutiontransmission electron microscopy (TEM) images of Pd—Pt core-shellparticles produced after 5 minutes in a deaerated 1 mM K₂PtCl₄ solutionare provided in FIGS. 4A-4B. The TEM images reveal that the higherintensity present around the edges of the nanoparticles reflects Ptdeposition on the Pd core. The TEM images also show the presence ofhighly anisotropic growth of metal deposits on carbon nanoparticles thatlead to the deposition of Pd nanowires.

The ORR activity and durability of the Pd/Pt core/shell nanoparticleswere measured. The results are provided in FIG. 5 which shows ORRpolarization curves for Pd solid nanoparticles with Pt shell prepared bythe Cu displacement method. The durability of the Pd/Pt nanoparticleswas tested with potential cycles swept between 0.6 V and 1.05 V at scanrate of 50 mVs⁻¹. No loss in surface area or ORR activity was observedfor Pd/Pt nanoparticles after 30,000 cycles and in fact cycling resultedin a small increase in activity and a shift of the E_(1/2) to >900 mV.Potential cycles pulsed between 0.65 V and 1.05 V with a 30-second dwelltime at each limit were used. Stepping between two limiting potentialswith long dwell time is considered to he a severe test of stabilitybecause the dissolution of low-coordinate sites is most rapid at 0.65 Vand defects are most likely regenerated above 1 V. This mechanism isbased on the reported highest dissolution rate of Pt(111) steps at 0.65V, and the 0.6-nm deep holes observed over the whole surface area at1.15 V. A very high activity of a Pt monolayer is achieved resulting inmass activity of 2.5 A/mg of Pt.

It will be appreciated by persons skilled in the art that the presentnanoparticles are not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of present nanoparticles isdefined by the claims which follow. It should further be understood thatthe above description is only representative of illustrative examples ofembodiments. For the reader's convenience, the above description hasfocused on a representative sample of possible embodiments, a samplethat teaches the principles of the present invention. Other embodimentsmay result from a different combination of portions of differentembodiments.

The description has not attempted to exhaustively enumerate all possiblevariations. That alternate embodiments may not have been presented for aspecific portion of the invention, and may result from a differentcombination of described portions, or that other undescribed alternateembodiments may be available for a portion, is not to be considered adisclaimer of those alternate embodiments. It will be appreciated thatmany of those undescribed embodiments are within the literal scope ofthe following claims, and others are equivalent. Furthermore, allreferences, publications, U.S. patents, and U.S. Patent ApplicationPublications cited throughout this specification are hereby incorporatedby reference in their entireties as if fully set forth in thisspecification.

1. A catalyst particle comprising: a carbon support having a highsurface area per unit mass; and an elongated metal nanoparticlecovalently bound to the carbon support.
 2. The catalyst particle ofclaim 1, wherein the elongated metal nanoparticle has a morphology of ananowire or a nanorod.
 3. The catalyst particle of claim 1, wherein theelongated metal nanoparticle comprises Pd.
 4. The catalyst particle ofclaim 1, wherein the elongated metal nanoparticle comprises a continuousand nonporous shell with a solid core.
 5. The catalyst particle of claim1, wherein the carbon support is functionalized.
 6. The catalystparticle of claim 5, wherein the functionalized carbon support comprisesa plurality of carbon nanoparticles having exposed on the surface afunctional group selected from carboxylic, carbonyl, phenol, lactone ora combination thereof.
 7. The catalyst particle of claim 4, wherein theshell is more noble than the core.
 8. The catalyst particle of claim 7,wherein the elongated metal nanoparticle comprises a Pd core and a Ptshell.
 9. The catalyst particle of claim 4, wherein the shell comprises4 to 12 monolayers of platinum (Pt).
 10. An electrode comprising: acarbon support having a high surface area per unit mass; and anelongated metal nanoparticle covalently bound to the carbon support. 11.The electrode of claim 10, wherein the elongated metal nanoparticle hasa morphology of a nanowire or a nanorod.
 12. The electrode of claim 10,wherein the elongated metal nanoparticle comprises Pd.
 13. The electrodeof claim 10, wherein the elongated metal nanoparticle comprises acontinuous and nonporous shell with a solid core.
 14. The electrode ofclaim 10, wherein the carbon support is functionalized.
 15. Theelectrode of claim 14, wherein the functionalized carbon supportcomprises a plurality of carbon nanoparticles having exposed on thesurface a functional group selected from carboxylic, carbonyl, phenollactone or a combination thereof.
 16. The electrode of claim 13, theshell is more noble than the core.
 17. The electrode of claim 16,wherein the elongated metal nanoparticle comprises a Pd core and a Ptshell.
 18. An energy conversion device comprising: a first electrode; aconducting electrolyte; and a second electrode, wherein at least one ofthe first or second electrodes comprises a plurality of catalystparticles of claim
 1. 19. The energy conversion device of claim 18,wherein the elongated metal nanoparticle has a morphology of a nanowireor a nanorod.
 20. The energy conversion device of claim
 18. wherein theelongated metal nanoparticle comprises Pd.
 21. The energy conversiondevice of claim 18, wherein the elongated metal nanoparticle comprises apalladium (Pd) core and platinum (Pt) shell having a shape of a nanowireor a nanorod with an external diameter of 3 nm to 9 nm.